PMT Window Research Articles

A novel multi-anode MCP-PMT with Cherenkov radiator window

With the proposal of the road map of Time-of-Flight positron emission tomography (TOF-PET) with a Coincidence Time Resolution (CTR) of 10 ps by Paul Lecoq et al. in 2020, the technology based on TOF puts forward higher requirements on the performance of its core optical devices. In this manuscript, we developed a kind of Multi-Anode micro channel plate photomultiplier tube (MCP-PMT) based on Cherenkov radiator window. By using the Cherenkov radiator as the optical window of Multi-Anode MCP-PMT directly, the optical interface between the scintillator and the PMT window in conventional TOF-PET is eliminated, reducing light loss and further improve the time resolution of TOF-PET. The anode of this MCP-PMT can be split into a finely spaced array of collecting electrodes to achieve good positional sensitivity. In this paper, the single photon detection capability and time resolution of this kind of MCP-PMT are evaluated. The transit time spread (TTS) at single photon mode can reach 42.9 ps, and the TTS at multi photon mode is less than 10 ps. The CTR based on this kind of MCP-PMT is 156.7 ± 10.6 ps FWHM, which is equivalent to a position resolution of 23.5 ± 1.6 mm along a line of response (LOR). The performance uniformity of this kind MCP-PMT has also been calibrated.

Discriminating the muon noise at the PMT windows in calorimeters

In particle physics experiments, stray muons are known to be a source of anomalous signals when they produce Cherenkov photons in the PMT window. Most recent of such events were reported by CMS Hadronic Forward calorimeters, where PMTs read out the Cherenkov signal generated within the quartz fibers. This study focuses on finding an efficient way to discriminate the Cherenkov signal coming from the quartz fibers from the ones generated at the face of the PMT due to stray muons. We report that these two different signals can be discriminated based on their pulse widths. The study also shows the performance of a proof-of-concept circuit prototype that effectively tags the two types of pulses.

Observed light yield of scintillation pixels: Extending the two-ray model

In this paper we propose an extended, two dimensional model describing the propagation of scintillation photons inside a cuboid crystal until they reach a PMT window. In the simplest approach the model considers two main reasons for light losses: standard absorption obeying the classical Lambert–Beer law and non-ideal reflectivity of the “mummy” covering formed by several layers of Teflon tape wrapping the sample. Results of the model calculations are juxtaposed with experimental data as well as with predictions of an earlier, one dimensional model.

Performance of HFGFlash at CMS

We report the performance of HF calorimeter and some details about the PMT window hit events. The new 4-anode PMTs will be installed during the 2013 shutdown period. We have developed HF GFlash, a very fast simulation of electromagnetic showers using parameterizations of the profiles in Hadronic Forward Calorimeter. HF GFlash has good agreement to 7 TeV Collision Data and previous Test Beam results. In addition to good agreement with Data and previous Test Beam results, HF GFlash can simulate about 10000 times faster than Geant4.

HF Status

We report the performance of HF calorimeter and some details about the PMT window hit events. The new 4-anode PMTs will be installed during the 2013 shutdown period. We have developed HF GFlash, a very fast simulation of electromagnetic showers using parameterizations of the profiles in Hadronic Forward Calorimeter. HF GFlash has good agreement to 7 TeV Collision Data and previous Test Beam results. In addition to good agreement with Data and previous Test Beam results, HF GFlash can simulate about 10000 times faster than Geant4.

Light Readout Optimisation using Wavelength Shifter - Reflector Combinations

The use of reflectors coated with a wavelength shifter (WLS) along with standard bialkali PMTs is an economical method for an efficient readout system for vacuum ultra violet (VUV) light produced in large liquid argon detectors. Various thicknesses of tetraphenyl butadiene (TPB) were deposited by spraying and vacuum evaporation onto both specular 3MTM-foil and diffuse TetratexTM (TTX) reflectors. 128 nm VUV light generated in 1 bar argon gas by a 5.4 MeV α source was detected by a 3-inch bialkali borosilicate PMT within a 1 m tube lined internally with a TPB coated reflector. The light collection was recorded as a function of separation between source and PMT for each combination of coating and reflector for distances up to 1m. Reflection coefficients of TPB coated reflectors were measured using a spectroradiometer. WLS coating on the PMT window was also studied. The optimum coating and reflector combination was TPB evaporated on TTX. Measurements with coating thicknesses of 0.2 mg/cm2 and 1.0 mg/cm2 yielded a similar performance. The best PMT window coating is obtained by TPB evaporation of 0.05 mg/cm2.

Study of Various Photomultiplier Tubes for Window Events: Upgrade R&D for CMS Hadron Forward Calorimeters

The PMTs of the CMS Hadron Forward calorimeters were found to generate a large amount of signal when their windows were traversed by energetic charged particles. This signal, which is due to Čerenkov light production at the PMT window, could interfere with the calorimeter signal and mislead the measurements. In order to find a viable solution to this problem, the response of different types of PMTs to muons traversing their windows at different orientations is measured at the H2 beam-line at CERN. Certain kinds of PMTs with thinner windows show significantly lower response to direct muon incidence. For one specific type - the four anode PMT- a simple and powerful algorithm to identify such events and recover the PMT signal using the signals of the quadrants without window hits is also presented. For the measurement of PMT responses to Čerenkov light, the Hadron Forward calorimeter signal was mimicked by two different setups in electron beams and the PMT performances were compared with each other. Superior performance of particular PMTs was observed.

Boron and thermal neutron interactions on borosilica window photomultiplier tubes

The borosilica, a very common PMT window and envelope material, contains 5% Boron (1% 10B). The high cross section for 10B capturing thermal neutrons (3980 barn), is a concern for LHC experiments using borosilica window PMTs. This study investigates the rate and the size of the signals generated by thermal neutron boron interaction in borosilica window PMTs; Hamamatsu R7525-HA and R7600U-200-M4. Although virtually all of the thermal neutrons incident on the borosilicate glass are absorbed, probability of generating a PMT signal was measured to be 3 × 10−4 and 3 × 10−6 for R7525-HA and R7600U-200-M4 PMTs, respectively. For these signals the average pulse size was found to be between 20–30 photoelectrons. We also discuss that four anode PMTs allow the elimination of these events with an offline algorithm.

Study of various photomultiplier tubes with muon beams and Čerenkov light produced in electron showers

The PMTs of the CMS Hadron Forward calorimeter were found to generate a large size signal when their windows were traversed by energetic charged particles. This signal, which is due to Čerenkov light production at the PMT window, could interfere with the calorimeter signal and mislead the measurements. In order to find a viable solution to this problem, the response of four different types of PMTs to muons traversing their windows at different orientations is measured at the H2 beam-line at CERN. Certain kinds of PMTs with thinner windows show significantly lower response to direct muon incidence. For the four anode PMT, a simple and powerful algorithm to identify such events and recover the PMT signal using the signals of the quadrants without window hits is also presented. For the measurement of PMT responses to Čerenkov light, the Hadron Forward calorimeter signal was mimicked by two different setups in electron beams and the PMT performances were compared with each other. Superior performance of particular PMTs was observed.

Scintillation yield of Bi 4Ge 3O 12 (BGO) pixel crystals

BGO crystals have been grown using the Czochralski method by the West Pomeranian University of Technology in Szczecin, Poland. We report the measurements of scintillation yield and low temperature thermoluminescence, performed on polished 2×2×10 mm 3 pixel and 2×2×2 mm 3 cube samples. The yield of the pixels placed horizontally on the PMT window, as well as of the cubes, is about twice higher than the yield of the pixels placed vertically. Such a distinct light loss is interpreted in the frame of a simple two-ray model, describing quantitatively the scintillation yield of pixel samples depending on the measurement geometry. Related to this effect, the common overuse of “BGO units” for expressing the light output of any scintillator is discussed. Thermoluminescence glow curves consist of several peaks attributed to the existence of traps, which turn out to be responsible for a slight decrease of the scintillation yield measured at room temperature in a short time window. To improve the performance of BGO crystals, efforts should be aimed at reducing both the internal light losses and the trap concentration, with the main emphasis laid on the former.

Development of wavelength shifter coated reflectors for the ArDM argon dark matter detector

To optimise the design of the light readout in the ArDM 1-ton liquid argon dark matter detector, a rangeof reflector and WLS coating combinations were investigated in several small setups, where argon scintillation light was generated by radioactive sources in gas at normal temperature and pressure and shifted into the blue region by tetraphenyl butadiene (TPB). Various thicknesses of TPB were deposited by spraying and vacuum evaporation onto specular3MTM-foil and diffuse Tetratex® (TTX) substrates. Light yields of each reflector and TPB coating combination were compared. Reflection coefficients of TPB coated reflectors were independentlymeasured using a spectroradiometer in a wavelength range between200 and 650 nm.WLS coating on the PMT window was also studied. These measurements were used to define the parameters of the light reflectorsof the ArDM experiment. Fifteen large 120 × 25 cm2 TTX sheetswere coated and assembled in the detector. Measurements in argon gas are reported providing good evidence of fulfilling the light collectionrequirements of the experiment.

Studies of a proximity focusing RICH with aerogel radiator for future Belle upgrade

We have been developing a proximity focusing ring imaging Cherenkov counter (RICH) with aerogel radiator, to extend the hadron identification capabilities in the forward endcap region of the Belle detector at the KEK B factory. This paper presents our recent studies on possible improvement of the RICH performance by using precise timing information. Cherenkov photons emitted in the aerogel radiator as well as in the entrance window of the PMT were used for the time-of-flight (TOF) measurement. A beam test using the Burle 85011 MCP PMT showed an excellent resolution of about 35 ps for Cherenkov photons from the PMT window. Such TOF measurement would supplement the RICH performance, and allow to positively identify particles below the Cherenkov threshold in aerogel, such as kaons and protons in the region around 1 GeV/ c.

Proximity focusing RICH with TOF capabilities

A proximity focusing RICH with aerogel radiator has been studied to extend the hadron identification capabilities in the forward region of the Belle spectrometer. Such a proximity focusing RICH counter is also a very fast detector if a micro-channel plate (MCP) PMT is used as the photon detector. With its excellent timing properties, the same device could also serve as a time-of-flight counter and thus supplement other identification methods. A prototype of this novel device using BURLE 85011 64-anode, microchannel plate PMT, was tested on the bench and in the test beam at KEK. A good separation of pions and protons was observed in the test beam data with a time-of-flight resolution of about 35 ps (rms) for Cherenkov photons produced in the PMT window.

Proximity focusing RICH with TOF capabilities

A proximity focusing RICH with aerogel radiator has been studied to extend the pion-kaon separation power in the forward region of the Belle spectrometer. Such a proximity focusing RICH counter is also a very fast detector, in particular if a micro-channel plate (MCP) PMT is used as the photon detector. With its excellent timing properties, the same device could also serve as a time-of-flight counter and thus supplement other identification methods, in particular for low momentum tracks. Cherenkov photons emitted in the radiator medium (aerogel) as well as in the entrance window of the PMT could be used for the time-of-flight measurement. A prototype of this novel device using BURLE 85011 64-anode, microchannel plate PMT, was tested on the bench and in the test beam at KEK. Excellent performance of this counter could be demonstrated. In particular, a good separation of pions and protons was observed in the test beam data with a time-of-flight resolution of about 35 ps (rms) for Cherenkov photons produced in the PMT window.

Construction and calibration of detectors for high-resolution metabolic breast cancer imaging

Each of two detectors used in our Positron Emission Mammography (PEM) system consists of four 36 mm × 36 mm × 20 mm bismuth germanate (BGO) crystal detector blocks coupled to a crossed-wire anode position-sensitive photomultiplier tube (PS-PMT). To facilitate high spatial-resolution imaging, the crystal blocks have been finely pixelated using a diamond saw. In each detector, 36 × 36 1.9 mm × 1.9 mm crystal elements are coupled directly to the PMT window and, on the opposite face of the blocks, 35 × 35 elements are offset by 1.0 mm along both the x- and y-axis of the PS-PMT. As part of a system calibration routine, a novel method for crystal element identification has been developed. This algorithm successfully identifies 59 × 49 crystal elements on each detector face. These results are used to generate a Look-Up-Table (LUT) that is accessed during image formation for the effective correction of spatial distortion inherent in the detectors. Crystal identification also facilitates the capability for accurate energy discrimination, since the detector gain is considered on an element-by-element basis by accesing an energy LUT. Employing a third LUT, which contains the relative efficiencies of individual crystal elements results in improvement in image uniformity from 50% to 13%.

YAlO 3: Ce-Am light pulsers as a gain monitor for undoped CsI detectors in a magnetic field

YAlO 3-Am pulsers show no degradation in their light output or resolution upon the application of a magnetic field, at least up to 10 kG. The decrease in the pulse height agrees with the change in the PMT gain calibrated by an LED. When the pulser is mounted on the top face of a 30 cm long CsI far from a PMT with a UV filter inserted between the PMT and the CsI, the light output decreases by a factor of 1 4 , compared with the case of being directly mounted on the PMT window. In the above configuration, the 5.49 MeV α of 241Am gives about 550 photoelectrons, which corresponds to a γ-equivalent energy of 6.6 MeV. Finally, the pulsers were mass-produced with a FWHM resolution of 5.4% (average) for the pulse height.